[SPARK-4765] Make GC time always shown in UI.

This commit removes the GC time for each task from the set of
optional, additional metrics, and instead always shows it for
each task.

cc pwendell

Author: Kay Ousterhout <kayousterhout@gmail.com>

Closes #3622 from kayousterhout/gc_time and squashes the following commits:

15ac242 [Kay Ousterhout] Make TaskDetailsClassNames private[spark]
e71d893 [Kay Ousterhout] [SPARK-4765] Make GC time always shown in UI.

core/src/main/resources/org/apache/spark/ui/static/webui.css

@@ -171,6 +171,6 @@ span.additional-metric-title {
 
 /* Hide all additional metrics by default. This is done here rather than using JavaScript to
  * avoid slow page loads for stage pages with large numbers (e.g., thousands) of tasks. */
-.scheduler_delay, .gc_time, .deserialization_time, .serialization_time, .getting_result_time {
+.scheduler_delay, .deserialization_time, .serialization_time, .getting_result_time {
   display: none;
 }

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -132,13 +132,6 @@ private[ui] class StagePage(parent: StagesTab) extends WebUIPage("stage") {
                   <span class="additional-metric-title">Task Deserialization Time</span>
                 </span>
               </li>
-              <li>
-                <span data-toggle="tooltip"
-                      title={ToolTips.GC_TIME} data-placement="right">
-                  <input type="checkbox" name={TaskDetailsClassNames.GC_TIME}/>
-                  <span class="additional-metric-title">GC Time</span>
-                </span>
-              </li>
               <li>
                 <span data-toggle="tooltip"
                       title={ToolTips.RESULT_SERIALIZATION_TIME} data-placement="right">
@@ -168,7 +161,7 @@ private[ui] class StagePage(parent: StagesTab) extends WebUIPage("stage") {
           ("Executor ID / Host", ""), ("Launch Time", ""), ("Duration", ""),
           ("Scheduler Delay", TaskDetailsClassNames.SCHEDULER_DELAY),
           ("Task Deserialization Time", TaskDetailsClassNames.TASK_DESERIALIZATION_TIME),
-          ("GC Time", TaskDetailsClassNames.GC_TIME),
+          ("GC Time", ""),
           ("Result Serialization Time", TaskDetailsClassNames.RESULT_SERIALIZATION_TIME),
           ("Getting Result Time", TaskDetailsClassNames.GETTING_RESULT_TIME)) ++
         {if (hasAccumulators) Seq(("Accumulators", "")) else Nil} ++
@@ -308,7 +301,7 @@ private[ui] class StagePage(parent: StagesTab) extends WebUIPage("stage") {
             <tr class={TaskDetailsClassNames.TASK_DESERIALIZATION_TIME}>
               {deserializationQuantiles}
             </tr>
-            <tr class={TaskDetailsClassNames.GC_TIME}>{gcQuantiles}</tr>,
+            <tr>{gcQuantiles}</tr>,
             <tr class={TaskDetailsClassNames.RESULT_SERIALIZATION_TIME}>
               {serializationQuantiles}
             </tr>,
@@ -429,7 +422,7 @@ private[ui] class StagePage(parent: StagesTab) extends WebUIPage("stage") {
             class={TaskDetailsClassNames.TASK_DESERIALIZATION_TIME}>
           {UIUtils.formatDuration(taskDeserializationTime.toLong)}
         </td>
-        <td sorttable_customkey={gcTime.toString} class={TaskDetailsClassNames.GC_TIME}>
+        <td sorttable_customkey={gcTime.toString}>
           {if (gcTime > 0) UIUtils.formatDuration(gcTime) else ""}
         </td>
         <td sorttable_customkey={serializationTime.toString}

core/src/main/scala/org/apache/spark/ui/jobs/TaskDetailsClassNames.scala

@@ -24,9 +24,8 @@ package org.apache.spark.ui.jobs
  * If new optional metrics are added here, they should also be added to the end of webui.css
  * to have the style set to "display: none;" by default.
  */
-private object TaskDetailsClassNames {
+private[spark] object TaskDetailsClassNames {
   val SCHEDULER_DELAY = "scheduler_delay"
-  val GC_TIME = "gc_time"
   val TASK_DESERIALIZATION_TIME = "deserialization_time"
   val RESULT_SERIALIZATION_TIME = "serialization_time"
   val GETTING_RESULT_TIME = "getting_result_time"